Composite structures such as those used in the aerospace industries may be fabricated using automated fiber placement (AFP) machines which laydown relatively narrow strips of preimpregnated tows or slit composite tape onto a manufacturing tool, such as a mandrel. AFP machines may include one or more tape application heads and are typically operated by NC (numeric control) or CNC (computer numeric control) controllers which control the movement of heads and ancillary functions, including placing and cutting the tows “on the fly”. In a typical AFP machine application, carbon fiber-epoxy tows are pulled from storage spools or creels through a set of guides. From the guides, the tows enter a cutter assembly where they are cut to the correct length by knives as the material course, also called a tow band, is laid over a substrate. The substrate may comprise a tool, mandrel or one or more underlying layers of composite material which have been previously laid down and compacted. Each tow has a dedicated cutting knife, however the number of knifes may vary depending on the number of tows and the width of each tow. As the tows emerge from the cutting assembly, they pass over a compaction roller which applies and compresses the tows onto the substrate surface. Tension may be maintained on the tows at all times to assist in pulling them through the AFP machine.
Accurate placement of tows during the laydown process is important in fabricating quality structures meeting high performance specifications. Tow placement accuracy may be challenging where the layup includes numerous short courses, ramps or other contoured features and/or bi-directional laydown. Placement accuracy also may be affected by the need for the application head to continuously accelerate in multiple axes during the laydown process, particularly where the speed of the laydown is adjusted on-the-fly. Finally, placement accuracy may be affected by the sequence and manner in which tows are drawn from the supply reels, fed and subsequently cut and compacted during a laydown sequence.
Currently, an essentially manual process may be used to measure the accuracy of tow placement and make adjustments to compensate for tow misplacements. For example, one process may begin with the laydown of bi-directional plies at slow, medium and fast speeds on a tool base. Then, any alternating laps or gaps between courses are manually measured. Any alternating laps or gaps are manually adjusted in the machine controller and then rerun. Any deviations in start placement or end placement for each tow of each course are manually measured and compared to scribe lines on the tool base. Any deviations in start or end placement are manually adjusted in the controller for each tow and then rerun. The steps described immediately above may be repeated until all laps and gaps between courses are minimized, and the placement of each tow is within a predetermined, acceptable distance of the scribe lines. The current process for correcting tow misplacements is therefore time consuming and may be subject to operator error.
Accordingly, there is a need for a method of compensating for misplacement of tows by an AFP machine that is both rapid and requires minimal operator intervention and/or manual measurements.